Collection of accurate geophysical property data is a key to successful exploration and production of petroleum resources. Based on data collected in a wellbore (such as electrical and nuclear properties), as well as the propagation of sound through a formation, geophysicists make many important operational decisions. For example, geophysicists may use wellbore data to select where to set casing in a well and how to perforate a well to stimulate hydrocarbon flow.
One method of collecting wellbore geophysical properties is by way of wireline well-logging. In wireline well-logging, a well-logging tool (also often referred to as a sonde) is lowered into a wellbore on an electrical cable, the wireline. The well-logging tool is an electrically powered measurement device that may, for example, collect electrical data, sonic waveforms that have propagated through the surrounding formation, or radioactivity counts. These measurements are usually converted to a digital form and transmitted on the wireline. Other methods of well-logging are known as logging while drilling (LWD) and measuring while drilling (MWD). In these types of well-logging, data are collected during the drilling operation.
Well-logging data are normally indexed by the depth at which the measurements were made. With modern equipment, the logging measurements may also be made based on a time interval rather than on a depth interval. Such time domain logs are ultimately converted into depth indexed data.
The accuracy of the data is a crucial element in the value of the well-log data. Data accuracy depends on both the accuracy of the measurements and the accuracy of the depth index.
In wireline logging the depth index is usually determined by measuring how much cable has been lowered into the borehole and measuring how much cable has been reeled back in. These measurements are done at the surface as the cable passes through an Integrated Depth Wheel (IDW). However, IDW measurements are prone to inaccuracies with respect to the actual depth of the measurement tools. For example, if the tools become stuck in the borehole (due to various borehole conditions) while the well is being logged in an uphole direction, the cable may stretch as the winch continues to reel in the cable. Similar problems also occur in LWD and MWD logging because, for example, coil tubing and drill pipe may become bent.
Several different approaches have been suggested for adjusting the recorded depth index. One approach described in U.S. Pat. No. 5,019,978, issued to Howard, Jr. et al., uses accelerometers to determine the location of the measurement tools. The accelerometer data are applied in an algorithm to correct the depth index. Another approach described in U.S. Pat. No. 6,704,655, issued to Kelly, uses tension meter data in combination with the accelerometer data in a proportional-integral-derivative control loop algorithm to correct the depth index. U.S. Pat. No. 7,020,557 issued to Reniska discloses a wireline well-logging data acquisition system and methods for establishing corrected depth based on pressure readings from two pressure sensors separated by a known distance.
Accordingly, there is still a need for better and improved methods for obtaining correct depth measurements and for controlling downhole equipment at accurate depth during downhole operations.